The effect of porcelain firing and type of finish line on the marginal fit of zirconia copings.

STATEMENT OF THE PROBLEM
Although all-ceramic restorations are broadly used, there is a lack of information concerning how their fit is affected by fabrication procedure and marginal configuration.


PURPOSE
The purpose of this study was to evaluate the marginal fit of zirconia CAD/CAM ceramic crowns before and after porcelain firing. The influence of finish line configuration on the marginal fit was also evaluated.


MATERIALS AND METHOD
Twenty standardized zirconia CAD/CAM copings were fabricated for chamfer and shoulder finish line designs (n=10). The marginal fit of specimens was measured on 18 points, marked on the master metal die by using a digital microscope. After the crowns were finalized by porcelain veneering, the measurements of marginal fit were performed again. The means and standard deviations were calculated and data were analyzed using student's t-test and paired t-test (α=0.05).


RESULTS
There were significant differences between marginal fit of chamfer and shoulder finish line groups before and after porcelain firing (p= 0.014 and p= 0.000, respectively). The marginal gap of copings with shoulder finish line was significantly smaller than those with chamfer configuration (p= 0.000), but there were no significant differences between the two marginal designs, after porcelain firing (p= 0.341).


CONCLUSION
Porcelain veneering was found to have a statistically significant influence on the marginal fit of zirconia CAD/CAM crowns. Both margin configurations showed marginal gaps that were within a reported clinically acceptable range of marginal discrepancy.


Introduction
As far as esthetics and acceptable biocompatibility of dental restoration are concerned, all-ceramic crowns have recently gained large popularity. If made of high quality, all-ceramic restorations are difficult to be distinguished from unrestored adjacent teeth. [1] Among the many ceramic systems that have been developed, [2][3] Yttria-stabilized polycrystalline tetrag-onal zirconia has become a popular form of dental restoration; mostly because of its notable characteristics including esthetics, excellent biocompatibility, low plaque accumulation, and high strength. [4] For the fabrication of zirconium oxide core, computer-aided design/ computer-aided manufacturing (CAD/CAM) is used by the system. [5] A compatible feldspathic translucent veneering porcelain (facing porcelain) is applied onto the white zirconia core to guarantee the excellent esthetics of the restorations. [4] This veneering process which includes a firing procedure (sintering) at high temperature (750-900°C) and subsequent cooling of the restoration, is carried out at least once, but usually 2-5 times. [6] One of the most important standards in clinical assessment and success of fixed dental restorations is marginal fit of the crown. [7][8][9][10] In fact; marginal misfit has many severe outcomes which may induce prospective failure of the prosthesis. [9] Large marginal discrepancies make the luting agent to be disclosed within the oral environment. If the marginal gap is large, the cement will decompose rapidly as a result of oral fluids and chemomechanical forces. [11] This microleakage, in part, results in secondary caries, pulpal inflammation, and necrosis. [9][10][11][12][13] Inept marginal adaptation also causes plaque retention and compositional changes in the subgingival microflora, and consequently inflammation in gingival and periodontal tissues. [14] Finally, marginal misfit generates stress concentrations which may decrease the strength of the restoration. [15] Marginal fit of the crown is defined as the gap between the prepared tooth and the intaglio surface of the restoration. Absolute marginal discrepancy is the linear distance between the cavosurface finish line of the preparation and the margin of the restoration. [16] This measurement displays the total misfit at the margin and is always considered as the largest measurement of the error at that point. [17] Mclean et al. defined clinically acceptable marginal discrepancies to be between 40 to 120 µm. [18] Previous studies have reported marginal discrepancy range of zirconia ceramic crowns to be 19 to 160 µm. [15,[19][20][21][22][23] However, there is limited studies on the marginal fit of zirconia-based materials in comparison with conventional ceramic or metal restorations. [24] Given the importance of the fitting accuracy of restoration, [4] there has been much debate on the effect of veneering porcelain on all-ceramic restorations fit. [15,[19][20][25][26][27] To name a few, Balkaya et al. reported that the porcelain firing cycle has an influence on the marginal fit of In-ceram all-ceramic crowns. [15] Castellani et al. also pointed out that the marginal area of single crowns manufactured with different all-ceramic systems deforms significantly during the porcelain ve-neering process. [26] Contrary to these findings, Pera et al. found that the processes of firing and glazing of vitadur-N veneer did not alter the dimensional stability of In-ceram substructures. [19] Moreover, the effect of the type of marginal design on the fitting precision of restoration should also be studied rigorously; for, there is no mutual agreement concerning ideal margin configuration of all-ceramic restorations. Researchers advocated either deep chamfer or rounded shoulder finish lines. [20,28] Some studies on Procera ceramics [29] and zirconia ceramic crowns [30] suggested a significant difference in marginal gap between the two marginal designs. Based on their findings, rounded shoulder was identified to perform better.
In contrast, some other studies illustrated that margin configuration had no significant difference on the marginal fit of ceramic crowns. [20,[31][32] Marginal discrepancy can, in fact, be measured by using several methods such as direct view of the crown on a die, cross-sectional view, impression replica technique, and clinical examination. [33] The direct view, as used by the researchers of the current study, is a nondestructive technique which is frequently employed to measure the distortion during the manufacturing process of the restorations. [15] The purpose of this study was to evaluate the marginal fit of zirconia CAD/CAM ceramic crowns before and after porcelain firing. The influence of finish line configuration on marginal fit was also evaluated.
The null hypothesis was that no differences would be found in the marginal fit of zirconia CAD/CAM crowns before and after porcelain firing, and among different finish lines.

Fabrication of master dies
Brass master dies (     ( Figure 3) Then, the measurements were taken based on the produced images.

Porcelain firing cycles
At this stage, the copings were prepared for porcelain application (Vita VM9; Vident, Germany). Porcelain application was done 0.5 mm short of margin. [24,33] A silicone index was used to standardize the shape and the size of veneers. Next, the dentin and enamel porce-  Table 1 shows the means and standard deviations for the marginal gap of the specimens before and after porcelain firing in micrometers, sorted out by the margin configuration.  Certainly, this study was not free of limitations.

Results
Some of these restrictions are discussed as follows.
First, marginal fit was measured in this experimental design; however, the internal fit of the crowns was not.
The reason was that measuring the internal fit of the crowns required the crowns to be cemented and the specimens to be sectioned. Second, all copings were produced and tested under ideal conditions, which may not reflect the conditions which can be seen in daily clinical practices. Third, the copings were not subjected to mechanical and thermal cycling; while thermo mechanical cycling is one of the most important factors which affects the long-term success of the restoration.
[ [41][42] Finally, although brass dies were used for measurement, use of human natural teeth would be more ideal.

Conclusion
Within the limitations of this study, the following conclusions could be drawn: 1. Porcelain veneering showed to have a statistically significant influence on the marginal fit of zirconia CAD/CAM crowns.
2. There were no significant differences between completed crowns of chamfer and shoulder margins.

Both margin configurations demonstrated marginal
gaps that were within a reported clinically acceptable range of marginal discrepancy.